In the rotary drilling of boreholes in earth formations, a drill bit is connected to the lower end of a hollow drill string which is lowered and rotated to cause the bit cutters to dislodge particles or cuttings from the hole bottom. Usually these bits have two or more shanks depending from a bit body and each shank supports a rotatably mounted cone cutter which faces toward the center of the borehole. A drilling fluid such as gas or liquid is circulated downwardly through the drill string and up the wall of the borehole to remove cuttings.
In drilling some of the hard abrasive formations, zones may be encountered where the formation is badly fractured and loose chunks may be left in the borehole wall as the bit drills through the zone.
These chunks are free to move and intrude into the borehole between the borehole wall and bit and cause wear and damage to the shanks or legs of the bit just above the cutters.
The presence of such zones have been noted in the Ocean Drilling Program. The Ocean Drilling program is a multinational geophysical research project. Core samples are taken from the ocean floor for analysis using techniques and equipment common to the oil and mining industries. Severe damage to their core bits occurred when they encountered badly fractured basaltic formations. The leading edges and outer surfaces of the shanks were worn away to the extent that o-rings protecting the cone bearing systems were exposed and destroyed causing premature bearing failure Some shanks were worn thin enough to cause shank breakage resulting in cone and bearing assemblies being left in the borehole. These assemblies had to be "fished" out of the borehole before drilling could continue.
Other instances of similar problems have occurred in drilling blast holes in taconite mines where the formation has been badly fractured from previous blasts. Large loose rock fragments falling in from the borehole wall against the bit as it rotates cause wear and damage to the shanks of rotary cone bits.
Rotary percussion bits which are also commonly used in the mining industry to drill blast holes experience similar wear and damage to the sides of the bit heads when drilling hard broken formations. This excessive wear on the sides of the head often lead to premature loss of the gage cutting inserts.
In previous attempts to address this problem in rotary cone mining bits, hardfacing pads and flat-top tungsten carbide inserts were placed in the shanks to minimize abrasive wear and damage. This was successful to a limited degree. Another method was tried unsuccessfully in which carbide balls were placed in a raceway on the shank and allowed to rotate as described in the Schumacher U.S. Pat. No. 3,130,801.
In the case of percussion bits, inserts were installed in the sides of the bits above each gage insert. This reduced the erosion of steel from above the gage inserts and extended the useful life of the bits.